Process of liquefying gases



I Sept. 15-, 1959 P. E. BOCQUET 2,903,858

PROCESS OF LIQUEFYING GASES Filed Oct. 6. 1955 TURBO EXPANDER TURBO 22 EXPANDER I6 2o 25 "c o 24 G RECYOLE GAS I8 \I 3s E F 30 38 H THROTTLING l-IWID VALVE FIG. I

PRESSURE r E c P LIQUID GAS FIG. 2

ENT HALPY H INVENTOR.

PHILIP E. BOGQUET BYw/ww ATTORNEY PROCESS or LIQUEFYING GASES 1 Philip E. Bocquet, Ponca City, Okla., assignor to Cons tock Liquid Methane Corporation, New York, N.Y., a corporation of Delaware Application October 6, 1955, Serial No. 538,975

Claims. (Cl. 62-11) This invention relates to improvements in the art of liquefying gases, and more particularly, to an improved process of liquefying gases by refrigeration.

In liquefying gas by one known method of refrigeration, the gas stream initially exists in a fluid state at an elevated pressure. The fluid stream is first chilled and then passed through a plurality of stages of expansion, where a portion of the stream is reduced to a liquid and the remainder exists in a gaseous state. The liquid and gaseous phases are then separated, and the gas stream is recycled through compressors and coolers for reversion to its initial state; whereupon the recycled stream is recombined with the feed stream for another passage through the expansion cycle.

The expansion of the stream 'is accomplished by directing the fluid through successive stages of a multiple-stage turboexpander, where the fluid is reduced in pressure in each stage. As the fluid is expanded into the two-phase region, liquid will form within the particular stage of the turboexpander, and the liquid will be injected with the gas into the succeeding stage of the turboexpander. A

The

similar action occurs in each subsequent stage. liquid has a lower velocity than the gas, therefore, the efliciency of each stage of the turboexpander receiving both gas and liquid is reduced. Also, the tip speed of the turboexpander blades exceeds the velocity of the liquid. Therefore, the tips of the blades run into the liquid-resulting in erosion of the blades and a reduced service life for the turboexpansion unit.

The present invention contemplates a novel method of liquefaction, wherein the liquid injected into a turboexpander will be reduced to a minimum. It is proposed to remove the liquid from the fluid stream between successive stages of the turboexpander when operating in the two-phase region, whereby the efficiency of the subsequent stages will be increased, and the work normally obtained from the expander is not substantially reduced.

An important object of this invention is to increase the service life of turboexpanders used in the'liquefaction of gases.

Another object of this invention is to increase the efliciency of turboexpanders used in the liquefaction of gases.

A further and more specific object of this invention is to substantially eliminate the injection of liquid into the second and subsequent stages of turboexpanders operating within the two-phase region and used in the liquefaction of gases by refrigeration.

Other objects and advantages of the invention will be evident from the following detailed description, when read in conjunction with the accompanying drawing, which illustrates my invention.

In the drawing:

Figure l is a flow diagram of my novel process of liquefaction.

Figure 2 is an enthalpy-pressure chart illustrating the changes in pressure and enthalpy of a fluid stream during the liquefaction of the stream by the present process.

States PatentO 2,903,858 Patented Sept. 15, 1959 ice Broadly stated, the present invention may be defined as the process of at least partial liquefaction-of a fluid stream by a reduction in the temperature and pressure thereof which comprises:

(a) Passing such stream through a turboexpander to eflect a partial reduction in pressure and temperature of the stream accompanied by apartial liquefaction thereof,

'(b) Separating the liquid, from the efiiuent of said expander,

(c) Passing the gaseous remainder of such efiluent through a second turboexpander to complete said reduction in temperature and pressure accompanied by a further liquefaction of said stream, and

(d) Separating the liquid from the eflluent of said second expander. p i Referring to the drawing in detail, and particularly Figure 1, reference character 4 designates a singlestage turboexpander of any suitable type which will expand a fluid stream injected through the inlet 6 thereof. In a commercial installation of the turboexpander 4, the expander will be connected to some mechanism whereby the work generated by operation of the expander 4 may be put to use. However, this feature forms no part of the present invention and is, therefore, not shown. The exhaust 8 of the turboexpander 4 is connected bya con duit 10 to the inlet 12' of a separator 14. The separator 14 may be of any suitable type which will effectively separate a gas-liquid stream into its separate gas and liquid components. In the type of separator shown, the

gas is directed into the upper portion of the separator and the liquid is directed into the lower portion of the separator. 1

The gas outlet 16 of the separator v14 is connected by a conduit 18 to the inlet 20 of another single-stage turbo; expander 22. The turboexpander 22 maybe of thesame type as the turboexpander 4 and, in the usual case, will be drivingly connected to any desired'apparatus whereby the work generated by the turboexpander 22 will be. effectively utilized. Another conduit 24 interconnects the exhaust 26 of the turboexpander 22 to another separator 28. Thus, the exhaust from the turboexpander 22 (which contains both gas and liquid) will be directed into the separator 28. p Another conduit 30 interconnects the lower outlet 32 of the-separator 14 to the conduit 24, whereby the bottom efiiuent from the separator 14 is by-passed around the turboexpander 22 and directed into the second separator 28. A throttling valve 34 is interposed in the conduit 30 to reduce the pressure of the bottom eflluent discharging from the first separator 14, as will be more fully hereinafter set forth.

The separator 28 may be of the same .type as the separator 14 and has an upper outlet 36 for the discharge of gas, aswell as a lower outlet 38 for the discharge of liquid.

In operation of the apparatus illustrated in Figure 1, the feed stream to be liquefied is directed to the inlet 6 of the turboexpander 4. As previously stated, the fluid stream at this point in a complete liquefaction process usually exists in a fluid state at an elevated pressure. As the stream is expanded in the turboexpander 4, the pressure of the stream will be reduced suificiently to form a two-phase mixture comprising a gas phase and a liquid phase. The two-phase mixture is discharged through the conduit 10 into the separator 14, where the gases will be directed upwardly into the upper portion of the separator 14 and the liquids will be directed downwardly into the lower portion ofthe separator. 1

The gases, which constitute the main portion of the original feed stream, are directed through the conduit 18 into the second-turboexpander 22 The gas will again be expanded in the turboexpander 22, to again reduce the pressure of the stream and form another two-phase mixture having a gaseous phase and a liquid phase. This two-phase mixture is in turn directed to the second separator 28. g

"The liquid collected in the separator 14 is discharged through the-conduit30 and is throttled by the valve 34, whereby the pressure of the liquid is reduced to a pressure corresponding to the discharge pressure of the second turboexpander 22. As the liquid is throttled through the valve 34, a portion of the liquid will be transformed to a gaseous state, to provide another two-phase mixture beingdirected into the second separator 28.

The two-phase mixtures entering the separator 28 will be effectively separated. In the embodiment shown, the gases are directed upwardly and the liquids downwardly, as in the separator 14. The upper outlet 36 of the separator 28 may be connected to another turboexpander (not shown) to provide a third expansion of the feed stream; or the discharging gas may be recycled through a'compression and cooling cycle (not shown) for recombining with the initial feed stream and repeated passage through the turboexpanders 4 and 22. The liquid discharging from the lower outlet 38 may be either passed through another throttling valve (not shown) and/or directed to a suitable storage (not shown), as desired. It will thus be apparent that the feed stream is passed throughtwo stages of turboexpansion, yet no liquid is injected into the second turboexpander. Therefore, the 'efficiency and service life of the second turboexpander 22 will be enhanced. Also, the liquid throttled through the valve 34 will not provide an appreciable loss of work, as will be'apparent from the following description and an examination of Figure 2.

The chart of Figure 2 represents a typical enthalpypressure curve for a gas, such as methane. The Roman numerals I and II represent the expansion of a gas by a multiple-stage turboexpander where the liquid is not separated from the stream between successive stages of expansion, with the line from I to B being one expansion and from B to II being a second expansion. The letters A through H in Figure 2 correspond to the points in the present process designated by the corresponding letters in Figure -1.

Thecurve of Figure 2 between I and B or A and B shows that the stream enters into two phases (gas and liquid) during the first stage of expansion of both the prior and present processes. The curve from B to II represents a second expansion by prior methods wherein no separation is performed between stages. The remaining curves are for the presentprocess. From B to C and B to E indicates the separation of the gas and liquid, respectively, in the separator 14. C to D is the expansioncurve'of the gas through the second turboexpander 22, 'andthe'curve from E to F symbolizes the throttling of the liquid through the valve 34.

' A study of the specific example illustrated in Figure 2 reveals that substantially the same amount of work may be recovered, and the same quantity of liquid may be finally obtained, by expansion of the stream in successive stages without liquid separation, as compared with expansion of the stream with separationand throttling of the liquid between successive stages. It will be understoodthat-the chart of Figure 2 is -a theoretical representation -of the pressure-enthalpy relationship of the "stream at various stages in the-process andassumes the same thermodynamic etficiency of the turboexpanders in both the prior and present processes. As a practical matter, -however, the absence of liquid in the second stage of turboexpansion (the turboexpander 22) will "appreciably "increase the efficiency of this-turboexpander. Therefore, a greater fracton of the fluid stream will be liquefied and more -workcan be recovered from the process, as contrasted with the process where the liquid is norremoved between successive stages of turboexpansion. "-In otherwords, substantially the same or an additional amount of work is recovered when only the gas phase of the stream is passed through the second stage of turboexpansion as when the complete stream is passed through the second stage. No appreciable work is lost by throttling the liquid since the specific volume of the liquid is small and does not increase as the pressure is decreased. Also, the work which could be recovered from the evolved gas during liquid throttling is small, since only a small amount is evolved before the pressure energy is expended.

From the foregoing, it will be apparent that the present invention will provide an increase in the service life of turboexpanders used in the liquefaction of gases. The present process is not limited to a two-stage expansion as illustrated in the drawings, but may be used with any number of expansion stages with the liquid being removed from the exhaust of each stage and throttled to the pressure of the stream at a later stage of expansion of the stream. The only liquid which will exist in the various stages of the turboexpanders will be the liquid provided by expansion of the stream in the particular stage. Therefore, erosion of the turboexpander blades in the second and subsequent stages will be materially reduced, and the over-all efficiency of the turboexpanders will be increased.

While particular embodiments of the invention have been described, it will be understood, of course, that the invention is not limited thereto since many modifications may be made, and it is, therefore, contemplated to cover by the appended claims any such modifications as fall within the true spirit and scope of the invention.

The invention having thus been described, what is claimed and desired to be secured by Letters Patent is:

1. The process of at least partial liquefaction of a fluid stream by a reduction in the temperature and pressure thereof which comprises: (a) passing such stream through a turbo-expander to effect a partial reduction in pressure and temperature of the stream accompanied by a partial liquefaction thereof, (b) separating the liquid from the efiluent of said expander, (c) passing the gaseous remainder of such eflluent .through a second turbo-expander to complete said reduction in temperatureand pressure accompanied by a further liquefaction of said stream, and (d) reducing the pressure of the liquid separated from the effiuent of said first expander to substantially that of the effluent of said second expander.

2. The process of at least partial liquefaction of a fluid stream by a reduction in the temperature and pressure thereof which comprises: (a) passing such stream through a turbo-expander to effect a partial reduction in pressure and temperature of the stream accompanied by a partial liquefaction thereof, (b) separating the liquid from the effluent of said expander, (c) passing .the gaseous remainder of such effluent through another turbo-expander to complete said reduction in temperature and pressure accompanied by a further liquefaction of said stream, ((2) reducing the pressure of the liquid separated from the efiiuent of said first expander to substantially that of the efiluent of said other expander, and (e) separating the liquid and gaseous phases of the diluent of said other expander.

3. The process of at least partial liquefaction of a fluid stream by a reduction in the temperature and pressure thereof which comprises: (a) passing such stream through a turbo-expander to effect a partial reduction in pressure and temperature of the stream accompanied by a partial liquefaction thereof, (b) separating the liquid from the effiuent of said expander, (c) passing the gaseous remainder of such eflluent through a second turbo-expander to complete said reduction in temperature and pressure accompanied by a further liquefaction of said stream, (d) reducing the pressure of the liquid separated from the effluent of said first expander to substantially that of the elfiuent of said second expander, (e) combining the efliuent of said pressure reduction with the effluent of said second expander, and (f) separating the liquid and gaseous phases of such composite.

4. The process of at least partially liquefying a fluid stream by a reduction in the temperature and pressure thereof which comprises (a) passing such stream through a turbo-expander to efiect a partial reduction in pressure and temperature of the stream accompanied by a partial liquefaction thereof, (b) separating the liquid from the effluent of said expander, (c) passing the gaseous remainder of such effluent through a second turboexpander for further reduction in temperature and pressure accompanied by a further liquefaction of said stream, (d) reducing the pressure of the liquid separated from the effluent of said first expander to substantially that of References Cited in the file of this patent UNITED STATES PATENTS 2,664,719 Rice et a1. Jan. 5, 1954 2,666,303 Schuftan Jan. 19, 1954 FOREIGN PATENTS 329,418 Great Britain May 22, 1930 

1. THE PROCESS OF AT LEAST PARTIAL LIQUEFACTION OF A FLUID STREAM BY A REDUCTION IN THE TEMPERATURE AND PRESSURE THEREOF WHICH COMPRISES: (A) PASSING SUCH STREAM THROUGH A TURBO-EXPANDER TO EFFECT A PARTIAL REDUCTION IN PRESSURE AND TEMPERATURE OF THE STREAM ACCOMPANIED BY A PARTIAL LIQUEFACTION THEREOF, (B) SEPARATING THE LIQUID FROM THE EFFLUENT OF SAID EXPANDER, (C) PASSING THE GASEOUS REMAINDER OF SUCH EFFLUENT THROUGH A SECOND 